Thyroxine (T-4) is found in serum as free thyroxine (free T-4) and as bound thyroxine (bound T-4). In its bound form, thyroxine is conjugatd with thyroxine binding proteins (TBP). The relative amount of free and bound T-4 present in serum depends on two factors; the amount of T-4 secreted by the thyroid and the concentration of TBP in the serum. The concentration of TBP may vary according to the condition of the subject. For instance, the presence of excess estrogens, such as in pregnancy and in the use of oral contraceptives, causes an increase in the TBP level. Thus, the use of total T-4 assays for the differential determination of hyperthyroid, euthyroid and hypothyroid conditions can lead to false diagnoses. However, the percentage of free T-4 in serum varies with changes in TBP levels. Therefore, the concentration of free T-4 in serum is an indicator of thyroid function which is unchanged even during wide variances in TBP levels.